Aperiodic csi reporting method based on aperiodic csi-rs in wireless communication system, and device therefor

ABSTRACT

Disclosed in the present application is a method by which a terminal reports aperiodic channel status information (CSI) to a base station in a wireless communication system. Particularly, the method comprises the steps of: receiving, from the base station, through downlink control information (DCI), a CSI report triggering message including information on at least one CSI process among a plurality of CSI processes and on one aperiodic reference signal among a plurality of aperiodic reference signal resources included in the at least one CSI process; and updating the aperiodic CSI relating to the at least one CSI process and reporting the same to the base station, on the basis of the one aperiodic reference signal.

TECHNICAL FIELD

The present disclosure relates to a wireless communication system, andmore particularly, to a method for reporting an aperiodic CSI (ChannelStatus Information) based on an aperiodic CSI-RS (Channel StatusInformation-Reference Signal) in a wireless communication system and toa device for performing the method.

BACKGROUND

3GPP LTE (3rd generation partnership project long term evolutionhereinafter abbreviated LTE) communication system is schematicallyexplained as an example of a wireless communication system to which thepresent invention is applicable.

FIG. 1 is a schematic diagram of E-UMTS network structure as one exampleof a wireless communication system. E-UMTS (evolved universal mobiletelecommunications system) is a system evolved from a conventional UMTS(universal mobile telecommunications system). Currently, basicstandardization works for the E-UMTS are in progress by 3GPP. E-UMTS iscalled LTE system in general. Detailed contents for the technicalspecifications of UMTS and E-UMTS refers to release 7 and release 8 of“3rd generation partnership project; technical specification group radioaccess network”, respectively.

Referring to FIG. 1, E-UMTS includes a user equipment (UE), an eNode B(eNB), and an access gateway (hereinafter abbreviated AG) connected toan external network in a manner of being situated at the end of anetwork (E-UTRAN). The eNode B may be able to simultaneously transmitmulti data streams for a broadcast service, a multicast service and/or aunicast service.

One eNode B contains at least one cell. The cell provides a downlinktransmission service or an uplink transmission service to a plurality ofuser equipments by being set to one of 1.25 MHz, 2.5 MHz, 5 MHz, 10 MHz,15 MHz, and 20 MHz of bandwidths. Different cells can be configured toprovide corresponding bandwidths, respectively. An eNode B controls datatransmissions/receptions to/from a plurality of the user equipments. Fora downlink (hereinafter abbreviated DL) data, the eNode B informs acorresponding user equipment of time/frequency region on which data istransmitted, coding, data size, HARQ (hybrid automatic repeat andrequest) related information and the like by transmitting DL schedulinginformation. And, for an uplink (hereinafter abbreviated UL) data, theeNode B informs a corresponding user equipment of time/frequency regionusable by the corresponding user equipment, coding, data size,HARQ-related information and the like by transmitting UL schedulinginformation to the corresponding user equipment. Interfaces foruser-traffic transmission or control traffic transmission may be usedbetween eNode Bs. A core network (CN) consists of an AG (access gateway)and a network node for user registration of a user equipment and thelike. The AG manages a mobility of the user equipment by a unit of TA(tracking area) consisting of a plurality of cells.

Wireless communication technologies have been developed up to LTE basedon WCDMA. Yet, the ongoing demands and expectations of users and serviceproviders are consistently increasing. Moreover, since different kindsof radio access technologies are continuously developed, a newtechnological evolution is required to have a future competitiveness.Cost reduction per bit, service availability increase, flexiblefrequency band use, simple structure/open interface and reasonable powerconsumption of user equipment and the like are required for the futurecompetitiveness.

DISCLOSURE Technical Purpose

Based on the above discussion, the present disclosure proposes a methodfor reporting aperiodic CSI based on aperiodic CSI-RS in a wirelesscommunication system and a device for such a method.

Technical Solution

In one aspect of the present disclosure, there is provided a method forreporting, by a user equipment (UE), aperiodic channel statusinformation (CSI) to a base station in a wireless communication system,the method comprising: receiving, from the base station, a CSIreport-triggering message via downlink control information (DCI),wherein the CSI report-triggering message include information about atleast one CSI process among a plurality of CSI processes, and about anaperiodic reference signal among a plurality of aperiodic referencesignal resources included in the at least one CSI process; and updatingthe aperiodic CSI for the at least one CSI process based on the singleaperiodic reference signal and reporting the updated aperiodic CSI tothe base station.

In another aspect of the present disclosure, there is provide a userequipment (UE) in a wireless communication system, the UE comprising: awireless communication module; and a processor coupled to the module,wherein the processor is configured for: receiving, from a base station,a CSI report-triggering message via downlink control information (DCI),wherein the CSI report-triggering message include information about atleast one CSI process among a plurality of CSI processes, and about anaperiodic reference signal among a plurality of aperiodic referencesignal resources included in the at least one CSI process; and updatingthe aperiodic CSI for the at least one CSI process based on the singleaperiodic reference signal and reporting the updated aperiodic CSI tothe base station.

In one embodiment, the processor is further configured for reporting, tothe base station, user equipment capability information including athreshold value of CSI computing capability of the UE, wherein when anumber of CSI reports to be updated for a predetermined time unitexceeds the threshold value, the processor is further configured forupdating only a number of CSI reports corresponding to a number smallerthan or equal to the threshold value. In one embodiment, when the numberof CSI reports to be updated for the predetermined time unit exceeds thethreshold value, the processor is configured for setting, to any value,a report value for a number of CSI reports corresponding to a number bywhich the exceeding number exceeds the threshold value. In oneembodiment, the processor is configured for first selecting, among anumber of CSI reports corresponding to a number smaller than or equal tothe threshold value, a CSI report corresponding to a lowest CSI processindex.

In one embodiment, the processor is configured for receiving informationabout the plurality of CSI processes including the plurality ofaperiodic reference signal resources via an upper layer from the basestation.

In one embodiment, the DCI include an uplink grant.

Technical Effect

According to an embodiment of the present disclosure, the user equipmentcan more efficiently report the aperiodic CSI based on the aperiodicCSI-RS in a wireless communication system.

It will be appreciated by persons skilled in the art that that theeffects that can be achieved through the present invention are notlimited to what has been particularly described hereinabove and otheradvantages of the present invention will be more clearly understood fromthe following detailed description.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a configuration of an Evolved Universal MobileTelecommunications System (E-UMTS) network as an example of a wirelesscommunication system.

FIG. 2 illustrates a control-plane protocol stack and a user-planeprotocol stack in a radio interface protocol architecture conforming toa 3rd Generation Partnership Project (3GPP) radio access networkstandard between a User Equipment (UE) and an Evolved UMTS TerrestrialRadio Access Network (E-UTRAN).

FIG. 3 illustrates physical channels and a general signal transmissionmethod using the physical channels in a 3GPP system.

FIG. 4 illustrates a structure of a radio frame in a Long Term Evolution(LTE) system.

FIG. 5 illustrates a structure of a downlink radio frame in the LTEsystem.

FIG. 6 illustrates a structure of an uplink subframe in the LTE system.

FIG. 7 is a schematic diagram of a typical multi-antenna (MIMO)communication system.

FIG. 8 shows an implementation of 2D-AAS.

FIG. 9 is a flow chart illustrating a method for receiving aperiodicCSI-RS and reporting aperiodic CSI based on the CSI-RS in accordancewith an embodiment of the present disclosure.

FIG. 10 is a block diagram of a communication apparatus according to anembodiment of the present invention.

DETAILED DESCRIPTIONS

The configuration, operation, and other features of the presentinvention will readily be understood with embodiments of the presentinvention described with reference to the attached drawings. Embodimentsof the present invention as set forth herein are examples in which thetechnical features of the present invention are applied to a 3rdGeneration Partnership Project (3GPP) system.

While embodiments of the present invention are described in the contextof Long Term Evolution (LTE) and LTE-Advanced (LTE-A) systems, they arepurely exemplary. Therefore, the embodiments of the present inventionare applicable to any other communication system as long as the abovedefinitions are valid for the communication system. In addition, whilethe embodiments of the present invention are described in the context ofFrequency Division Duplexing (FDD), they are also readily applicable toHalf-FDD (H-FDD) or Time Division Duplexing (TDD) with somemodifications.

The term ‘Base Station (BS)’ may be used to cover the meanings of termsincluding Remote Radio Head (RRH), evolved Node B (eNB or eNode B),Reception Point (RP), relay, etc.

FIG. 2 illustrates control-plane and user-plane protocol stacks in aradio interface protocol architecture conforming to a 3GPP wirelessaccess network standard between a User Equipment (UE) and an EvolvedUMTS Terrestrial Radio Access Network (E-UTRAN). The control plane is apath in which the UE and the E-UTRAN transmit control messages to managecalls, and the user plane is a path in which data generated from anapplication layer, for example, voice data or Internet packet data istransmitted.

A PHYsical (PHY) layer at Layer 1 (L1) provides information transferservice to its higher layer, a Medium Access Control (MAC) layer. ThePHY layer is connected to the MAC layer via transport channels. Thetransport channels deliver data between the MAC layer and the PHY layer.Data is transmitted on physical channels between the PHY layers of atransmitter and a receiver. The physical channels use time and frequencyas radio resources. Specifically, the physical channels are modulated inOrthogonal Frequency Division Multiple Access (OFDMA) for Downlink (DL)and in Single Carrier Frequency Division Multiple Access (SC-FDMA) forUplink (UL).

The MAC layer at Layer 2 (L2) provides service to its higher layer, aRadio Link Control (RLC) layer via logical channels. The RLC layer at L2supports reliable data transmission. RLC functionality may beimplemented in a function block of the MAC layer. A Packet DataConvergence Protocol (PDCP) layer at L2 performs header compression toreduce the amount of unnecessary control information and thusefficiently transmit Internet Protocol (IP) packets such as IP version 4(IPv4) or IP version 6 (IPv6) packets via an air interface having anarrow bandwidth.

A Radio Resource Control (RRC) layer at the lowest part of Layer 3 (orL3) is defined only on the control plane. The RRC layer controls logicalchannels, transport channels, and physical channels in relation toconfiguration, reconfiguration, and release of radio bearers. A radiobearer refers to a service provided at L2, for data transmission betweenthe UE and the E-UTRAN. For this purpose, the RRC layers of the UE andthe E-UTRAN exchange RRC messages with each other. If an RRC connectionis established between the UE and the E-UTRAN, the UE is in RRCConnected mode and otherwise, the UE is in RRC Idle mode. A Non-AccessStratum (NAS) layer above the RRC layer performs functions includingsession management and mobility management.

DL transport channels used to deliver data from the E-UTRAN to UEsinclude a Broadcast Channel (BCH) carrying system information, a PagingChannel (PCH) carrying a paging message, and a Shared Channel (SCH)carrying user traffic or a control message. DL multicast traffic orcontrol messages or DL broadcast traffic or control messages may betransmitted on a DL SCH or a separately defined DL Multicast Channel(MCH). UL transport channels used to deliver data from a UE to theE-UTRAN include a Random Access Channel (RACH) carrying an initialcontrol message and a UL SCH carrying user traffic or a control message.Logical channels that are defined above transport channels and mapped tothe transport channels include a Broadcast Control Channel (BCCH), aPaging Control Channel (PCCH), a Common Control Channel (CCCH), aMulticast Control Channel (MCCH), a Multicast Traffic Channel (MTCH),etc.

FIG. 3 illustrates physical channels and a general method fortransmitting signals on the physical channels in the 3GPP system.

Referring to FIG. 3, when a UE is powered on or enters a new cell, theUE performs initial cell search (S301). The initial cell search involvesacquisition of synchronization to an eNB. Specifically, the UEsynchronizes its timing to the eNB and acquires a cell Identifier (ID)and other information by receiving a Primary Synchronization Channel(P-SCH) and a Secondary Synchronization Channel (S-SCH) from the eNB.Then the UE may acquire information broadcast in the cell by receiving aPhysical Broadcast Channel (PBCH) from the eNB. During the initial cellsearch, the UE may monitor a DL channel state by receiving a DownLinkReference Signal (DL RS).

After the initial cell search, the UE may acquire detailed systeminformation by receiving a Physical Downlink Control Channel (PDCCH) andreceiving a Physical Downlink Shared Channel (PDSCH) based oninformation included in the PDCCH (S302).

If the UE initially accesses the eNB or has no radio resources forsignal transmission to the eNB, the UE may perform a random accessprocedure with the eNB (S303 to S306). In the random access procedure,the UE may transmit a predetermined sequence as a preamble on a PhysicalRandom Access Channel (PRACH) (S303 and S305) and may receive a responsemessage to the preamble on a PDCCH and a PDSCH associated with the PDCCH(S304 and S306). In the case of a contention-based RACH, the UE mayadditionally perform a contention resolution procedure.

After the above procedure, the UE may receive a PDCCH and/or a PDSCHfrom the eNB (S307) and transmit a Physical Uplink Shared Channel(PUSCH) and/or a Physical Uplink Control Channel (PUCCH) to the eNB(S308), which is a general DL and UL signal transmission procedure.Particularly, the UE receives Downlink Control Information (DCI) on aPDCCH. Herein, the DCI includes control information such as resourceallocation information for the UE. Different DCI formats are definedaccording to different usages of DCI.

Control information that the UE transmits to the eNB on the UL orreceives from the eNB on the DL includes a DL/UL ACKnowledgment/NegativeACKnowledgment (ACK/NACK) signal, a Channel Quality Indicator (CQI), aPrecoding Matrix Index (PMI), a Rank Indicator (RI), etc. In the 3GPPLTE system, the UE may transmit control information such as a CQI, aPMI, an RI, etc. on a PUSCH and/or a PUCCH.

FIG. 4 illustrates a structure of a radio frame used in the LTE system.

Referring to FIG. 4, a radio frame is 10 ms (327200×T_(s)) long anddivided into 10 equal-sized subframes. Each subframe is 1 ms long andfurther divided into two slots. Each time slot is 0.5 ms (15360×T_(s))long. Herein, T_(s) represents a sampling time and T₅=1/(15kHz×2048)=3.2552×10⁻⁸ (about 33 ns). A slot includes a plurality ofOrthogonal Frequency Division Multiplexing (OFDM) symbols or SC-FDMAsymbols in the time domain by a plurality of Resource Blocks (RBs) inthe frequency domain. In the LTE system, one RB includes 12 subcarriersby 7 (or 6) OFDM symbols. A unit time during which data is transmittedis defined as a Transmission Time Interval (TTI). The TTI may be definedin units of one or more subframes. The above-described radio framestructure is purely exemplary and thus the number of subframes in aradio frame, the number of slots in a subframe, or the number of OFDMsymbols in a slot may vary.

FIG. 5 illustrates exemplary control channels included in a controlregion of a subframe in a DL radio frame.

Referring to FIG. 5, a subframe includes 14 OFDM symbols. The first oneto three OFDM symbols of a subframe are used for a control region andthe other 13 to 11 OFDM symbols are used for a data region according toa subframe configuration. In FIG. 5, reference characters R1 to R4denote RSs or pilot signals for antenna 0 to antenna 3. RSs areallocated in a predetermined pattern in a subframe irrespective of thecontrol region and the data region. A control channel is allocated tonon-RS resources in the control region and a traffic channel is alsoallocated to non-RS resources in the data region. Control channelsallocated to the control region include a Physical Control FormatIndicator Channel (PCFICH), a Physical Hybrid-ARQ Indicator Channel(PHICH), a Physical Downlink Control Channel (PDCCH), etc.

The PCFICH is a physical control format indicator channel carryinginformation about the number of OFDM symbols used for PDCCHs in eachsubframe. The PCFICH is located in the first OFDM symbol of a subframeand configured with priority over the PHICH and the PDCCH. The PCFICHincludes 4 Resource Element Groups (REGs), each REG being distributed tothe control region based on a cell Identity (ID). One REG includes 4Resource Elements (REs). An RE is a minimum physical resource defined byone subcarrier by one OFDM symbol. The PCFICH is set to 1 to 3 or 2 to 4according to a bandwidth. The PCFICH is modulated in Quadrature PhaseShift Keying (QPSK).

The PHICH is a physical Hybrid-Automatic Repeat and request (HARQ)indicator channel carrying an HARQ ACK/NACK for a UL transmission. Thatis, the PHICH is a channel that delivers DL ACK/NACK information for ULHARQ. The PHICH includes one REG and is scrambled cell-specifically. AnACK/NACK is indicated in one bit and modulated in Binary Phase ShiftKeying (BPSK). The modulated ACK/NACK is spread with a Spreading Factor(SF) of 2 or 4. A plurality of PHICHs mapped to the same resources forma PHICH group. The number of PHICHs multiplexed into a PHICH group isdetermined according to the number of spreading codes. A PHICH (group)is repeated three times to obtain diversity gain in the frequency domainand/or the time domain.

The PDCCH is a physical DL control channel allocated to the first n OFDMsymbols of a subframe. Herein, n is 1 or a larger integer indicated bythe PCFICH. The PDCCH occupies one or more CCEs. The PDCCH carriesresource allocation information about transport channels, PCH andDL-SCH, a UL scheduling grant, and HARQ information to each UE or UEgroup. The PCH and the DL-SCH are transmitted on a PDSCH. Therefore, aneNB and a UE transmit and receive data usually on the PDSCH, except forspecific control information or specific service data.

Information indicating one or more UEs to receive PDSCH data andinformation indicating how the UEs are supposed to receive and decodethe PDSCH data are delivered on a PDCCH. For example, on the assumptionthat the Cyclic Redundancy Check (CRC) of a specific PDCCH is masked byRadio Network Temporary Identity (RNTI) “A” and information about datatransmitted in radio resources (e.g. at a frequency position) “B” basedon transport format information (e.g. a transport block size, amodulation scheme, coding information, etc.) “C” is transmitted in aspecific subframe, a UE within a cell monitors, that is, blind-decodes aPDCCH using its RNTI information in a search space. If one or more UEshave RNTI “A”, these UEs receive the PDCCH and receive a PDSCH indicatedby “B” and “C” based on information of the received PDCCH.

FIG. 6 illustrates a structure of a UL subframe in the LTE system.

Referring to FIG. 6, a UL subframe may be divided into a control regionand a data region. A Physical Uplink Control Channel (PUCCH) includingUplink Control Information (UCI) is allocated to the control region anda Physical uplink Shared Channel (PUSCH) including user data isallocated to the data region. The middle of the subframe is allocated tothe PUSCH, while both sides of the data region in the frequency domainare allocated to the PUCCH. Control information transmitted on the PUCCHmay include an HARQ ACK/NACK, a CQI representing a downlink channelstate, an RI for Multiple Input Multiple Output (MIMO), a SchedulingRequest (SR) requesting UL resource allocation. A PUCCH for one UEoccupies one RB in each slot of a subframe. That is, the two RBsallocated to the PUCCH are frequency-hopped over the slot boundary ofthe subframe. Particularly, PUCCHs with m=0, m=1, and m=2 are allocatedto a subframe in FIG. 6.

Hereinafter, a MIMO system will be described. MIMO refers to a method ofusing multiple transmission antennas and multiple reception antennas toimprove data transmission/reception efficiency. Namely, a plurality ofantennas is used at a transmitting end or a receiving end of a wirelesscommunication system so that capacity can be increased and performancecan be improved. MIMO may also be referred to as ‘multi-antenna’ in thisdisclosure.

MIMO technology does not depend on a single antenna path in order toreceive a whole message. Instead, MIMO technology collects datafragments received via several antennas, merges the data fragments, andforms complete data. The use of MIMO technology can increase systemcoverage while improving data transfer rate within a cell area of aspecific size or guaranteeing a specific data transfer rate. MIMOtechnology can be widely used in mobile communication terminals andrelay nodes. MIMO technology can overcome the limitations of therestricted amount of transmission data of single antenna based mobilecommunication systems.

The configuration of a general MIMO communication system is shown inFIG. 7.

A transmitting end is equipped with N_(T) transmission (Tx) antennas anda receiving end is equipped with N_(R) reception (Rx) antennas. If aplurality of antennas is used both at the transmitting end and at thereceiving end, theoretical channel transmission capacity increasesunlike the case where only either the transmitting end or the receivingend uses a plurality of antennas. Increase in channel transmissioncapacity is proportional to the number of antennas, thereby improvingtransfer rate and frequency efficiency. If a maximum transfer rate usinga signal antenna is R_(o), a transfer rate using multiple antennas canbe theoretically increased by the product of the maximum transfer rateR_(o) by a rate increment R_(i). The rate increment R_(i) is representedby the following equation 1 where R_(i) is the smaller of N_(T) andN_(R).

R _(i)=min(N _(T) , N _(R))   [Equation 1]

For example, in a MIMO communication system using four Tx antennas andfour Rx antennas, it is possible to theoretically acquire a transferrate four times that of a single antenna system. After theoreticalincrease in the capacity of the MIMO system was first demonstrated inthe mid-1990s, various techniques for substantially improving datatransfer rate have been under development. Several of these techniqueshave already been incorporated into a variety of wireless communicationstandards including, for example, 3rd generation mobile communicationand next-generation wireless local area networks.

Active research up to now related to MIMO technology has focused upon anumber of different aspects, including research into information theoryrelated to MIMO communication capacity calculation in various channelenvironments and in multiple access environments, research into wirelesschannel measurement and model derivation of MIMO systems, and researchinto space-time signal processing technologies for improvingtransmission reliability and transfer rate.

To describe a communication method in a MIMO system in detail, amathematical model thereof is given below. As shown in FIG. 7, it isassumed that N_(T) Tx antennas and N_(R) Rx antennas are present. In thecase of a transmission signal, a maximum number of transmittable piecesof information is N_(T) under the condition that N_(T) Tx antennas areused, so that transmission information can be represented by a vectorrepresented by the following equation 2:

s=[s₁,s₂, . . . , s_(N) _(T) ]^(T)   [Equation 2]

Meanwhile, individual transmission information pieces s₁,s₂, . . . ,s_(N) _(T) may have different transmission powers. In this case, if theindividual transmission powers are denoted by P₁,P₂, . . . , P_(N) _(T), transmission information having adjusted transmission powers can berepresented by a vector shown in the following equation 3:

ŝ=[ŝ₁,ŝ₂, . . . , ŝ_(N) _(T) ]^(T)=[P₁s₁,P₂s₂, . . . , P_(N) _(T) s_(N)_(T) ]^(T)   [Equation 3]

The transmission power-controlled transmission information vector Ŝ maybe expressed as follows, using a diagonal matrix P of a transmissionpower:

$\begin{matrix}{\hat{s} = {{\begin{bmatrix}P_{1} & \; & \; & 0 \\\; & P_{2} & \; & \; \\\; & \; & \ddots & \; \\0 & \; & \; & P_{N_{T}}\end{bmatrix}\begin{bmatrix}s_{1} \\s_{2} \\\vdots \\s_{N_{T}}\end{bmatrix}} = {Ps}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

NT transmission signals x₁,x₂, . . . , x_(N) _(T) to be actuallytransmitted may be configured by multiplying the transmissionpower-controlled information vector ŝ by a weight matrix w. In thiscase, the weight matrix is adapted to properly distribute transmissioninformation to individual antennas according to transmission channelsituations. The transmission signals x₁,x₂, . . . , x_(N) _(T) can berepresented by the following Equation 5 using a vector X. In Equation 5,W_(ij) is a weight between the i-th Tx antenna and the j-th informationand W is a weight matrix, which may also be referred to as a precodingmatrix.

$\begin{matrix}{x = {\quad{\begin{bmatrix}x_{1} \\x_{2} \\\vdots \\x_{i} \\\vdots \\x_{N_{T}}\end{bmatrix} = {{\begin{bmatrix}w_{11} & w_{12} & \ldots & w_{1\; N_{T}} \\w_{21} & w_{22} & \ldots & w_{2\; N_{T}} \\\vdots & \; & \ddots & \; \\w_{i\; 1} & w_{i\; 2} & \ldots & w_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\w_{N_{T}1} & w_{N_{T}2} & \ldots & w_{N_{T}N_{T}}\end{bmatrix}\begin{bmatrix}{\hat{s}}_{1} \\{\hat{s}}_{2} \\\vdots \\{\hat{s}}_{j} \\\vdots \\{\hat{s}}_{N_{T}}\end{bmatrix}} = {{W\; \hat{s}} = {WPs}}}}}} & \left\lbrack {{Equation}\mspace{14mu} 5} \right\rbrack\end{matrix}$

Generally, the physical meaning of a rank of a channel matrix may be amaximum number of different pieces of information that can betransmitted in a given channel. Accordingly, since the rank of thechannel matrix is defined as the smaller of the number of rows orcolumns, which are independent of each other, the rank of the matrix isnot greater than the number of rows or columns A rank of a channelmatrix H, rank(H), is restricted as follows.

rank(H)≤min(N _(T) ,N _(R))   [Equation 6]

Each unit of different information transmitted using MIMO technology isdefined as a ‘transmission stream’ or simply ‘stream’. The ‘stream’ maybe referred to as a ‘layer’. The number of transmission streams is notgreater than a rank of a channel which is a maximum number of differentpieces of transmittable information. Accordingly, the channel matrix Hmay be indicted by the following Equation 7:

# of streams≤rank(H)≤min(N _(T) ,N _(R))   [Equation 7]

where ‘# of streams’ denotes the number of streams. It should be notedthat one stream may be transmitted through one or more antennas.

There may be various methods of allowing one or more streams tocorrespond to multiple antennas. These methods may be described asfollows according to types of MIMO technology. The case where one streamis transmitted via multiple antennas may be called spatial diversity,and the case where multiple streams are transmitted via multipleantennas may be called spatial multiplexing. It is also possible toconfigure a hybrid of spatial diversity and spatial multiplexing.

Hereinafter, the reference signal will be described in more detail.

In general, for the channel measurement, a reference signal alreadyknown to both the transmitting side and the receiving side istransmitted from the transmitting side to the receiving side togetherwith the data. This reference signal informs the modulation technique aswell as the channel measurement, thereby to allow performing thedemodulation process. The reference signal is divided into a dedicatedreference signal (DRS) to the base station and a specific userequipment, that is, a user equipment-specific reference signal, and acommon reference signal (common RS or Cell-specific RS; CRS), which is acell-specific reference signal for all user equipments in the specificcell. Further, the cell-specific reference signal includes a referencesignal by which the UE measures CQI/PMI/RI and reports measurements tothe base station. This reference signal may be referred to as CSI-RS(Channel State Information-RS).

The above-mentioned CSI-RS has been proposed for channel measurement forPDSCH separately from the CRS. Unlike the CRS, the CSI-RS may be definedusing up to 32 different resource configurations to reduce inter-cellinterference (ICI) in a multi-cell environment.

The CSI-RS (resource) configurations differ according to the number ofantenna ports. The CSI-RS defined using a maximum number of differentresource configurations is transmitted between adjacent cells. UnlikeCRS, CSI-RS supports up to 8 antenna ports. In the 3GPP standardsdocument, a total of eight antenna ports from antenna ports #15 to #22are assigned to antenna ports for CSI-RS. Following Tables 1 and 2 showthe CSI-RS configuration defined in the 3GPP standards document.Particularly, Table 1 relates to the case of Normal CP, while Table 2shows the case of Extended CP.

TABLE 1 CSI Number of CSI reference signals configured reference 1 or 24 8 signal n_(s) n_(s) n_(s) config- (k′, mod (k′, mod (k′, mod urationl′) 2 l′) 2 l′) 2 Frame 0 (9, 5) 0 (9, 5) 0 (9, 5) 0 structure 1 (11,2)  1 (11, 2)  1 (11, 2)  1 type 1 2 (9, 2) 1 (9, 2) 1 (9, 2) 1 and 2 3(7, 2) 1 (7, 2) 1 (7, 2) 1 4 (9, 5) 1 (9, 5) 1 (9, 5) 1 5 (8, 5) 0 (8,5) 0 6 (10, 2)  1 (10, 2)  1 7 (8, 2) 1 (8, 2) 1 8 (6, 2) 1 (6, 2) 1 9(8, 5) 1 (8, 5) 1 10 (3, 5) 0 11 (2, 5) 0 12 (5, 2) 1 13 (4, 2) 1 14 (3,2) 1 15 (2, 2) 1 16 (1, 2) 1 17 (0, 2) 1 18 (3, 5) 1 19 (2, 5) 1 Frame20 (11, 1)  1 (11, 1)  1 (11, 1)  1 structure 21 (9, 1) 1 (9, 1) 1(9, 1) 1 type 2 22 (7, 1) 1 (7, 1) 1 (7, 1) 1 only 23 (10, 1)  1 (10,1)  1 24 (8, 1) 1 (8, 1) 1 25 (6, 1) 1 (6, 1) 1 26 (5, 1) 1 27 (4, 1) 128 (3, 1) 1 29 (2, 1) 1 30 (1, 1) 1 31 (0, 1) 1

TABLE 2 CSI Number of CSI reference signals configured reference 1 or 24 8 signal n_(s) n_(s) n_(s) config- (k′, mod (k′, mod (k′, mod urationl′) 2 l′) 2 l′) 2 Frame 0 (11, 4)  0 (11, 4)  0 (11, 4) 0 structure 1(9, 4) 0 (9, 4) 0  (9, 4) 0 type 1 2 (10, 4)  1 (10, 4)  1 (10, 4) 1 and2 3 (9, 4) 1 (9, 4) 1  (9, 4) 1 4 (5, 4) 0 (5, 4) 0 5 (3, 4) 0 (3, 4) 06 (4, 4) 1 (4, 4) 1 7 (3, 4) 1 (3, 4) 1 8 (8, 4) 0 9 (6, 4) 0 10 (2, 4)0 11 (0, 4) 0 12 (7, 4) 1 13 (6, 4) 1 14 (1, 4) 1 15 (0, 4) 1 Frame 16(11, 1)  1 (11, 1)  1 (11, 1) 1 structure 17 (10, 1)  1 (10, 1)  1(10, 1) 1 type 2 18 (9, 1) 1 (9, 1) 1  (9, 1) 1 only 19 (5, 1) 1 (5, 1)1 20 (4, 1) 1 (4, 1) 1 21 (3, 1) 1 (3, 1) 1 22 (8, 1) 1 23 (7, 1) 1 24(6, 1) 1 25 (2, 1) 1 26 (1, 1) 1 27 (0, 1) 1

In the above Table 1 and Table 2, (k′,l′) represents a RE index, k′represents a subcarrier index, l′ represents a OFDM symbol index. FIG.11 illustrates the CSI-RS configuration #0 in the case of the normal CPamong the CSI-RS configurations defined in the current 3GPP standarddocument.

Further, a CSI-RS subframe configuration may be defined. The CSI-RSsubframe configuration may be composed of a periodicity (T_(CSI-RS)) anda subframe offset (Δ_(CSI-RS)) expressed in a subframe unit. Table 3below shows the CSI-RS subframe configuration defined in the 3GPPstandard document.

TABLE 3 CSI-RS periodicity CSI-RS subframe offset CSI-RS-SubframeConfigT_(CSI-RS) Δ_(CSI-RS) I_(CSI-RS) (subframes) (subframes) 0-4 5I_(CSI-RS)  5-14 10 I_(CSI-RS)-5 15-34 20 I_(CSI-RS)-15 35-74 40I_(CSI-RS)-35  75-154 80 I_(CSI-RS)-75

Currently, the information about the ZP (zero-power) CSI-RS is includedin a form shown in Table 4 below into the CSI-RS-Config-r10 message andis transmitted via the RRC layer signal. In particular, the ZP CSI-RSresource configuration may be composed of zeroTxPowerSubframeConfig-r10and zeroTxPowerResourceConfigList-r10, which is a 16-bit sized bitmap.Among them, the zeroTxPowerSubframeConfig-r10 informs, via acorresponding I_(CSI-RS) value in Table 3, the periodicity and thesubframe offset at which the corresponding ZP CSI-RS is transmitted.Further, the zeroTxPowerResourceConfigList-r10 may be information thatinforms the ZP CSI-RS configuration. Each element of the bitmapindicates configurations included in a column corresponding to fourantenna ports for the CSI-RS in the above Table 1 or Table 2. That is,according to the 3GPP standard document, the ZP CSI-RS is defined onlywhen there are four antenna ports for the CSI-RS.

TABLE 4 -- ASN1START CSI-RS-Config-r10 ::= SEQUENCE { csi-RS-r10 CHOICE{ ... } zeroTxPowerCSI-RS-r10 CHOICE { release NULL, setup SEQUENCE {zeroTxPowerResourceConfigList-r10 BIT STRING (SIZE (16)),zeroTxPowerSubframeConfig-r10 INTEGER (0..154) } } } -- ASN1STOP

Now, a description of a Channel status information (CSI) report isgiven. In the current LTE standard, a MIMO transmission scheme iscategorized into open-loop MIMO operated without CSI and closed-loopMIMO operated based on CSI. Especially, according to the closed-loopMIMO system, each of the eNB and the UE may be able to performbeamforming based on CSI to obtain a multiplexing gain of MIMO antennas.To obtain CSI from the UE, the eNB allocates a PUCCH or a PUSCH tocommand the UE to feedback CSI for a downlink signal.

CSI is divided into three types of information: a Rank Indicator (RI), aPrecoding Matrix Index (PMI), and a Channel Quality Indicator (CQI).First, RI is information on a channel rank as described above andindicates the number of streams that can be received via the sametime-frequency resource. Since RI is determined by long-term fading of achannel, it may be generally fed back at a cycle longer than that of PMIor CQI. Second, PMI is a value reflecting a spatial characteristic of achannel and indicates a precoding matrix index of the eNB preferred bythe UE based on a metric of Signal-to-Interference plus Noise Ratio(SINR). Lastly, CQI is information indicating the strength of a channeland indicates a reception SINR obtainable when the eNB uses PMI.

For reference, according to the current 3GPP standard document, the CQIindex, the corresponding modulation order, and the coding rate are shownin Table 5 below.

TABLE 5 CQI code rate × index modulation 1024 efficiency 0 out of range1 QPSK 78 0.1523 2 QPSK 120 0.2344 3 QPSK 193 0.3770 4 QPSK 308 0.6016 5QPSK 449 0.8770 6 QPSK 602 1.1758 7 16QAM 378 1.4766 8 16QAM 490 1.91419 16QAM 616 2.4063 10 64QAM 466 2.7305 11 64QAM 567 3.3223 12 64QAM 6663.9023 13 64QAM 772 4.5234 14 64QAM 873 5.1152 15 64QAM 948 5.5547

In one example, the operation for calculating the CQI using theinterference measurement is as follows.

The user equipment needs to calculate the SINR as a factor necessary forcalculating the CQI. In this case, the UE may perform the receptionpower measurement (S-measure) of the desired signal using the RS such asNZP CSI-RS. For an interference power measurement (I-measure orInterference measurement), the UE measures the power of an interferingsignal resulting from removal of the desired signal from the receivedsignal.

The subframe sets C_(CSI.0) and C_(CSI.1) for CSI measurement may beconfigured via a higher layer signaling. The subframes corresponding toeach of the subframe sets may not overlap with each other, but may beincluded in only one set. In such a case, the UE may perform theS-measure using RS such as CSI-RS without special subframe restrictions.However, in the case of I-measure, the UE individually performs theI-measure for each of subframe sets C_(CSI.0) and C_(CSI.1). Thus, theUE has to perform two different CQI calculations for the subframe setsC_(CSI.0) and C_(CSI.1).

Recently, active research on the introduction of Active Antenna System(AAS) is under way in the next generation mobile communication. In theAAS, each antenna is composed of an active antenna including an activecircuit. Thus, varying the antenna pattern depending on the situationmay allow reducing interference or performing beamforming efficiently.

When such an AAS is constructed in two dimensions, that is, when 2D-AASis implemented, it is possible to more aggressively change thetransmission beam according to the position of the receiving end byadjusting the main lobe of the antenna three-dimensionally moreefficiently in terms of antenna patterns.

FIG. 8 shows an implementation of 2D-AAS. Specifically, in FIG. 8, it isassumed that the antenna array is a co-polarized antenna array with eachantenna element having the same polarization. Referring to FIG. 8, inthe 2D-AAS, the antennas are arranged in the vertical and horizontaldirections, and, thus, a system of multiple antennas can be constructed.

In a full dimension (FD)-MIMO system using the 2D-AAS, the base stationmay configure several CSI-RS resources in one CSI process for the UE. Inthis connection, the CSI process refers to an operation of feedbackingchannel information using an independent feedback configuration.

In this case, the UE does not regard the CSI-RS resources configured inthe single CSI process as an independent channel. Rather, the UEaggregates the resources and thus assumes a single large CSI-RSresource. Then, the UE computes and feeds back CSI from these resources.For example, the base station may configure three 4-port CSI-RSresources in a single CSI process for the UE. The UE aggregates theseresources and thus assumes a single 12-port CSI-RS resource. The UEcalculates and feeds back the CSI using the 12-port PMI from this CSI-RSresource. This reporting mode may be referred to as Class A CSIreporting in the LTE-A system.

Alternatively, the UE may assume that each CSI-RS resource is anindependent channel The UE selects one of the CSI-RS resources andcalculates and reports the CSI based on the selected resource. That is,the UE selects a CSI-RS with the strongest channel among the eightCSI-RSs. Then, the UE calculates the CSI based on the selected CSI-RSand reports the calculated CSI to the base station. In this regard, theUE reports the selected CSI-RS to the base station via the CSI-RSResource Indicator (CRI). For example, if the channel of the firstCSI-RS corresponding to T (0) is strongest, the UE configures CRI=0 andreports the same to the base station. This reporting mode may bereferred to as Class B CSI reporting in the LTE-A system.

To effectively demonstrate this feature, in the class B, the followingvariables may be defined for the CSI process. K is the number of CSI-RSresources in the CSI process. Nk denotes the number of CSI-RS ports ofthe k-th CSI-RS resource.

Recently, in 3GPP standardization, in addition to a periodic NZP(Nonzero Power) CSI-RS (Periodic CSI-RS; P CSI-RS) transmitted based onthe period and offset as shown in Table 3, an aperiodic NZP CSI-RS(Aperiodic CSI-RS; AP CSI-RS) has been introduced. In particular, the APCSI-RS differs from the P CSI-RS in that only one transmission thereofis performed at a specific point in time. Hereinafter, “NZP” is omitted.Unless “ZP” (zero power) is specified, the CSI-RS means the NZP CSI-RS.

More specifically, multiple CSI-RS resources may be configured in asingle CSI process via RRC layer signaling as an upper layer signaling.In this connection, several CSI-RS resources may be composed of only PCSI-RS or alternatively may be composed only of AP CSI-RS.Alternatively, multiple CSI-RS resources may be configured as acombination of P CSI-RS and AP CSI-RS.

When a single AP CSI-RS is configured in a single CSI process, UEoperation is clear as follows. When AP CSI reporting of the CSI processis triggered via the UL grant, the UE finds the AP CSI-RS in thesubframe (SF) on which the UL grant is received and measures the channelof the AP CSI-RS, and calculates CSI based on the measurement. The UEthen reports the CSI via the PUSCH after n subframes (where n=4 or 5)from the triggering time. Since there is no subframe configurationinformation in the configuration of AP CSI-RS configured via RRCsignaling, the UE receives the AP CSI-RS on the corresponding subframeon which the triggering occurs.

First Embodiment

The first embodiment of the present disclosure proposes a base stationoperation and a UE operation when a plurality of AP CSI-RSs areconfigured in a single CSI process. In particular, following two casesmay be distinguished from each other: a case when only a plurality of APCSI-RSs are configured in the single CSI process; and a case when asingle AP CSI-RS and at least a single CSI-RS are simultaneouslyconfigured in the single CSI process.

First, when a plurality of AP CSI-RSs are configured in a single CSIprocess, the base station operation and the UE operation are defined asone of 1) to 3) as follows.

1) The base station triggers AP CSI reporting of the corresponding CSIprocess via DCI (Downlink Control Information) or uplink grant. The UEperforms channel estimation from each of a plurality of AP CSI-RSsconfigured in the CSI process, selects one AP CSI-RS that is the mostpreferred. Then, the UE reports an index of the selected AP CSI-RS(i.e., the CSI-RS resource indicator (CRI)) and the RI/PMI/CQI for theselected AP CSI-RS to the base station.

2) The base station triggers AP CSI reporting of the corresponding CSIprocess via the DCI or uplink grant. The UE performs channel estimationfrom each of a plurality of AP CSI-RSs configured in the correspondingCSI process and reports RI/PMI/CQI for each channel to the base station.

3) The base station triggers a corresponding CSI process and an AP CSIreport for one of the plurality of AP CSI-RSs defined in the CSI processvia DCI or uplink grant. The UE checks the corresponding CSI process andthe single AP CSI-RS selected by the base station. The UE reports theRI/PMI/CQI for the AP CSI-RS to the base station. More specifically, thetriggering information included in the uplink grant of the DCI mayindicate a CSI process for AP CSI reporting and a single AP CSI-RS(defined within the CSI process). Further, combinations between the CSIprocess and the AP CSI-RS may be provided to the UE via RRC signaling inadvance by the base station. Triggering of the AP CSI report may bedefined by the base station such that one of the correspondingcombinations is indicated via the DCI, i.e., the uplink grant.

For example, the base station may configure a plurality of CSI processesfor the UE, via RRC signaling. Further, the base station may triggeraperiodic CSI via DCI or uplink grant. In this case, the base stationmay designate at least one of the plurality of CSI processes. In thiscase, according to the embodiment of the present disclosure, since aplurality of AP CSI-RS resources may be defined in the CSI process, thebase station may operate as follows: when the base station designatesthe at least one CSI process, the base station designates one of theplurality of AP CSI-RS resources together with the at least one CSIprocess.

The base station does not transmit the AP CSI-RS for the remaining APCSI-RS REs not triggered in the CSI process. Instead, the base stationuses the remaining AP CSI-RS REs that is not triggered in the CSIprocess for PDSCH transmission or mutes them. When the non-triggeredremaining AP CSI-RS RE is used for PDSCH transmission, a rate matchingsuch that the data may be mapped to the corresponding RE may be appliedto the UE receiving the corresponding data. When the remaining unappliedAP CSI-RS RE is muted, a rate matching may be applied to a UE receivingdata on an RB that includes the muted RE, such that the data is nottransmitted using the corresponding muted RE.

In one example, the UE expects the base station to configure asubset/superset relationship as established between a plurality of APCSI-RSs configured in a single CSI process. That is, when AP CSI-RS #1,AP CSI-RS #2 and AP CSI-RS #3 exist in the single CSI process, theantenna port and RE of the AP CSI-RS #3 may be respectively composed ofa subset of antenna ports and a subset of REs of the AP CSI-RS #2.Further, the antenna port and RE of AP CSI-RS #2 may be respectivelycomposed of a subset of antenna ports and a subset of REs of the APCSI-RS #1.

Next, in the first embodiment of the present disclosure, when at leastone AP CSI-RS and at least one P CSI-RS are simultaneously configured ina single CSI process, the base station operation and the UE operationare defined as one of the following operations.

a) The base station triggers AP CSI reporting of the corresponding CSIprocess via the uplink grant. The UE performs channel estimation fromeach of a plurality of AP CSI-RSs and a P CSI-RSs configured in acorresponding CSI process. The UE selects one most preferred AP CSI-RSand selects one most preferred P CSI-RS. The distinction between the APCSI-RS and the P CSI-RS may be grasped based on the presence or absenceof the subframe configuration information. That is, the CSI-RS withoutthe subframe configuration information is AP CSI-RS, while the CSI-RSwith the subframe configuration information is P CSI-RS. Alternatively,it may be determined whether the CSI-RS is an AP CSI-RS or a P CSI-RS,via a separate explicit indicator. Then, the UE reports the index (i.e.,CRI) of the selected AP CSI-RS and the RI/PMI/CQI for the selected APCSI-RS to the base station. Further, the UE reports the index (i.e.,CRI) of the selected P CSI-RS and the RI/PMI/CQI for the selected PCSI-RS to the base station.

b) The base station triggers AP CSI reporting of the corresponding CSIprocess via the uplink grant. The UE performs channel estimation fromeach of a plurality of AP CSI-RSs and a P CSI-RSs configured in thecorresponding CSI process. The UE selects one most preferred CSI-RS.Then, the UE reports the index (i.e., CRI) of the selected CSI-RS andthe RI/PMI/CQI for the selected CSI-RS to the BS. The selected CSI-RSmay be an AP CSI-RS or a P CSI-RS.

c) The base station triggers AP CSI reporting of the corresponding CSIprocess via the uplink grant. The UE performs channel estimation fromeach of a plurality of AP CSI-RSs and a P CSI-RSs configured in thecorresponding CSI process. The UE selects one most preferred P CSI-RS.Then, the UE reports the index (i.e., CRI) of the selected P CSI-RS andthe RI/PMI/CQI for the selected P CSI-RS to the base station. The UEreports the RI/PMI/CQI for each of all of the AP CSI-RSs to the basestation without selecting one most preferred AP CSI-RS.

d) The base station triggers the corresponding CSI process and the APCSI report for one of the CSI-RSs defined in the CSI process, via theuplink grant. The UE checks the corresponding CSI process and the singleCSI-RS selected by the base station. The UE reports the RI/PMI/CQI forthe selected CSI-RS to the base station. For the remaining CSI-RS REsnot triggered in the CSI process, the base station does not transmit theCSI-RS. Instead, the remaining CSI-RS REs not triggered in the CSIprocess are used for PDSCH transmission or are muted. When thenon-triggered remaining AP CSI-RS RE is used for PDSCH transmission, arate matching may be applied to the UE receiving the data such that thedata may be mapped to the corresponding RE. When the remainingnon-triggered AP CSI-RS RE is muted, a rate matching may be applied to aUE receiving data on an RB that includes the muted RE such that the datais not transmitted on the corresponding muted RE.

e) The base station triggers the AP CSI report for the corresponding CSIprocess via the uplink grant. Additionally, the base station signals, tothe UE, which of the AP CSI-RS and the P CSI-RS in the CSI process forwhich the AP CSI report is to be triggered.

When triggering for AP CSI-RS is instructed, the UE selects one of theAP CSI-RSs defined in the corresponding CSI process. The UE reports theselected AP CSI-RS index (i.e., CRI) and the RI/PMI/CQI for the selectedAP CSI-RS to the base station. Alternatively, the UE reports to the basestation the RI/PMI/CQI for each of all of the AP CSI-RSs defined in thecorresponding CSI process.

When triggering for P CSI-RS is instructed, the UE selects one of the PCSI-RSs defined in the corresponding CSI process. The UE reports theselected P CSI-RS index (i.e., CRI) and the RI/PMI/CQI for the selectedP CSI-RS to the base station. For the AP CSI-RS RE not triggered in theCSI process, the base station does not transmit the CSI-RS. Instead, theAP CSI-RS RE not triggered in the CSI process is used for the PDSCHtransmission or is mutated. When remaining non-triggered AP CSI-RS RE isused for PDSCH transmission, a rate matching may be applied to the UEreceiving the data so that the data may be mapped to the correspondingRE. When the remaining non-triggered AP CSI-RS RE is muted, a ratematching may be applied to a UE receiving data on an RB that includesthe muted RE, such that the data is not transmitted on the correspondingmuted RE.

Second Embodiment

In one example, when multiple AP CSI-RSs may be configured within asingle CSI process, or AP CSI-RS and P CSI-RS are configured together ina single CSI process, the base station operation and UE operation maybecome more complex. Therefore, in order to simplify this operation,restrictions may be imposed on the CSI process definition in the APCSI-RS configuration as follows.

When the base station is configuring the AP CSI-RS, the UE expects theAP CSI-RSs to be defined in different single CSI processes or the UEexpects that only a specific AP CSI-RS exists in the CSI process inwhich the specific AP CSI-RS exists. That is, the UE expects that aplurality of AP CSI-RSs will not be configured in a single CSI process.Alternatively, UE expects that AP CSI-RS and P CSI-RS are not definedtogether in a single CSI process. That is, the UE expects that the K (inthis connection, K is the number of CSI-RS defined in the class B CSIprocess) of the CSI process in which the AP CSI-RS exists is always setto one.

However, the above scheme may greatly limit the degree of freedom ofscheduling and operation of the base station. In order to allow thefreedom, it may be allowed that the multiple AP CSI-RSs are configuredwithin a single CSI process, while it may not be allowed that AP CSI-RSand P CSI-RS are configured simultaneously in a single CSI process.Alternatively, conversely, multiple AP CSI-RSs may be not allowed to beconfigured in a single CSI process, while AP CSI-RS and P CSI-RS may beallowed to be configured in a single CSI process at the same time.

Third Embodiment

According to the current 3GPP LTE standard, a plurality of CSI processesmay be configured for a single serving cell. In this connection, whenthe AP CSI report is triggered by the base station, the UE considersfollowing operations to reduce the CSI calculation amount.

The UE has the ability to calculate CSIs for up to Nx CSI processes.When the number of CSI processes for which CSI has not yet been reportedis defined as Nu, the UE calculates the CSIs only for the max(N_(x)−N_(u), 0) CSI processes among the CSI processes for which the APCSI reporting is triggered. The UE does not calculate the CSI for theremaining CSI processes.

When the AP CSI-RS is introduced later, in the above conventionaloperation, the UE may preferentially perform the CSI calculation for theCSI process in which the AP CSI-RS is configured. For example, whenN_(x)=4 and N_(u)=3, and when AP CSI reporting for CSI process #1 withAP CSI-RS configured therein and AP CSI reporting for CSI process #2with P CSI-RS configured therein are triggered simultaneously, the UEcalculates the CSI for the CSI process #1 and does not calculate the CSIfor the CSI process #2.

In another example, when N_(x)=4, and N_(u)=3 and the P CSI-RS isconfigured in the three reported CSI processes, and when the AP CSIreporting for the CSI process #1 with the AP CSI-RS configured thereinand the AP CSI reporting for the CSI process #2 with the AP CSI-RSconfigured therein are simultaneously triggered, the UE calculates theCSIs for the CSI processes #1 and #2. The UE does not compute the CSIfor one of the three non-reported CSI processes (e.g., a CSI processwith the highest index).

A case when the AP CSI reporting for multiple P CSI-RSs is triggered maybe compared with a case when the AP CSI reporting for multiple APCSI-RSs is triggered. The latter case requires higher UE computationalcomplexity than the former case. This is because of the followingreasons: in the former case, the UE may initiate the CSI calculationusing the P CSI-RS present at the time point prior to the triggering,while in the latter case, the UE must measure the AP CSI-RS and startthe CSI calculation at the triggering time.

In order to mitigate this high computational complexity, the UE maydetermine and supply the UE capability to the base station to indicate amaximum number of (e.g., m) of AP CSI-RSs for which the UE can deal withthe triggering of the CSI reports at a certain time point. When the basestation triggers CSI reports for the number of AP CSI-RSs larger than mat the certain time point, the UE calculates CSIs only for up to m APCSI-RSs and updates CSIs. The UE does not perform the CSI updates forthe number of AP CSI-RSs corresponding to a number by which theexceeding number exceeds the threshold value m (naturally, the UE mayperform CSI reporting for all of the AP CSI-RSs).

When there are K AP CSI-RSs in a single CSI process and the UE reports aCSI for a single AP CSI-RS selected via CRI, the CSI report is made onlyfor the selected single AP CSI-RS even when a plurality of AP CSI-RSsexist in the single CSI process. Thus, this may be defined as the UEreceiving the triggering for the CSI report for one AP CSI-RS. In yetanother aspect, even in this case, since the UE must measure all of theK AP CSI-RSs, this may be defined as the UE receiving triggering for CSIreporting on the K AP CSI-RSs with considering the processing capabilityof the UE.

When, in consideration of the limited CSI calculation capability by theUE, it is determined which of the AP CSI-RSs for the UE to update andwhich AP CSI-RSs for the UE not to update, the AP CSI-RS existing in theCSI process having the low index is preferentially updated. When the CSIprocess indices are the same, the AP CSI-RS with a lower AP CSI-RS indexis preferentially updated.

Alternatively, the UE may have the ability to compute CSI for up to m APCSI-RSs. Thus, the UE reports this capability to the base station as UEperformance information. In this connection, the number of AP CSI-RSsfor which CSIs has not yet been reported to the BS is defined as N. Inorder to reduce the computational complexity of the AP CSI for the APCSI-RS, the UE calculates the CSI for only the max (m-N_(u), 0) APCSI-RSs among the AP CSI-RSs for which the AP CSI report has beentriggered. The UE does not calculate and/or update the CSI for theremaining AP CSI-RSs.

Additionally, for relaxation of high computational complexity, CSIprocessing relaxation for the AP CSI-RS may be considered. In oneexample, when the AP CSI report for the AP CSI-RS has been triggeredduring the N sub-frames, the UE does not perform an update for AP CSIsexceeding K. N and K may be signaled by the base station to the UE, orthe UE may report N and K as the UE capability to the base station. In amore specific example, when K is 1 and when the base station hastriggered AP CSI for one AP CSI-RS at the time point n, and when thebase station has triggered the AP CSIs for additional AP CSI-RSs until alater time point n+N−1, the UE does not perform the update on the APCSIs for the additional AP CSI-RSs.

In one embodiment, the AP CSI-RS is transmitted once independentlyunlike the P CSI-RS. Thus, a current AP CSI-RS is independent of aprevious AP CSI-RS before the triggering of the AP CSI report for thecurrent AP CSI-RS. Therefore, it may be inappropriate for the UE not toperform the CSI update because there is no CSI reported prior to thetriggering time.

Thus, when the AP CSI reporting for AP CSI-RS has been triggered duringthe N sub-frames, the UE may report any CSI among a number of AP CSIscorresponding to the number exceeding K. As a result, the UE does notcalculate the CSI when the corresponding condition is satisfied, andrather, the UE selects and reports any CSI to the BS. Thus, the basestation interprets this CSI as a meaningless CSI. Alternatively, the UEdoes not expect the base station to trigger more than K AP CSI reportsfor AP CSI-RSs during N sub-frames. N may be fixed to 5 with consideringthat the minimum period of the existing P CSI-RS is 5 ms.

In order to further mitigate the high computational complexity, the UEmay determine and supply the UE capability to the base station toindicate a maximum number of (e.g., m) of AP CSI-RSs for which the UEcan deal with the triggering of the CSI reports at a certain time point.Thus, the UE may expect that the base station may not trigger CSIreports for the number of AP CSI-RSs larger than m at the certain timepoint. The base station receives the m value via the UE performancereport from the UE. Thereafter, the base station does not trigger theCSI reports for the number of AP CSI-RSs corresponding to a number bywhich the exceeding number exceeds the threshold value m to thecorresponding UE at a certain time.

FIG. 9 is a flow chart illustrating a method for receiving an aperiodicCSI-RS and reporting an aperiodic CSI according to an embodiment of thepresent disclosure.

Referring to FIG. 9, at operation 901, the UE may report UE capabilityinformation to the base station in advance, for example, in establishinga connection with the base station. In this case, the UE capabilityinformation may include a threshold value for the CSI calculationcapability.

Next, at operation 903, the UE may receive information regarding aplurality of CSI processes including a plurality of aperiodic CSI-RSresources via an upper layer, for example, an RRC layer. In other words,each CSI process may include a plurality of aperiodic CSI-RS resources.Further, each CSI process may also include one or more periodic CSI-RSresources.

Next, at operation 905, the UE receives a CSI report-triggering messagefrom the base station via the DCI or uplink grant. The CSIreport-triggering message includes information about a single CSIprocess among the plurality of CSI processes, and information on asingle aperiodic reference signal among the plurality of aperiodicreference signal resources included in the single CSI process. That is,the single CSI process and the single aperiodic reference signal arejoint-encoded and included in the CSI report-triggering message. Themessage may be provided to the UE via the DCI or uplink grant.

Finally, at operation 907, the UE updates and reports the aperiodic CSIfor the at least one CSI process to the base station based on the singleaperiodic reference signal.

However, when the number of CSI reports to be updated for apredetermined time unit exceeds the threshold value, only the number ofCSI reports corresponding to a number below the threshold number may beupdated. A value for the number of CSIs reports corresponding to anumber by which the exceeding number exceeds the threshold value may beconfigured to any value. In this case, among the number of CSI reportscorresponding to the number below the threshold, a CSI reportcorresponding to the lowest CSI process index is first selected.

FIG. 10 is a block diagram of a communication apparatus according to anembodiment of the present invention.

Referring to FIG. 10, a communication apparatus 1000 includes aprocessor 1010, a memory 1020, an RF module 1030, a display module 1040,and a User Interface (UI) module 1050.

The communication device 1000 is shown as having the configurationillustrated in FIG. 10, for the convenience of description. Some modulesmay be added to or omitted from the communication apparatus 1000. Inaddition, a module of the communication apparatus 1000 may be dividedinto more modules. The processor 1010 is configured to performoperations according to the embodiments of the present inventiondescribed before with reference to the drawings. Specifically, fordetailed operations of the processor 1010, the descriptions of FIGS. 1to 9 may be referred to.

The memory 1020 is connected to the processor 1010 and stores anOperating System (OS), applications, program codes, data, etc. The RFmodule 1030, which is connected to the processor 1010, upconverts abaseband signal to an RF signal or downconverts an RF signal to abaseband signal. For this purpose, the RF module 1030 performsdigital-to-analog conversion, amplification, filtering, and frequencyupconversion or performs these processes reversely. The display module1040 is connected to the processor 1010 and displays various types ofinformation. The display module 1040 may be configured as, not limitedto, a known component such as a Liquid Crystal Display (LCD), a LightEmitting Diode (LED) display, and an Organic Light Emitting Diode (OLED)display. The UI module 1050 is connected to the processor 1010 and maybe configured with a combination of known user interfaces such as akeypad, a touch screen, etc.

The embodiments of the present invention described above arecombinations of elements and features of the present invention. Theelements or features may be considered selective unless otherwisementioned. Each element or feature may be practiced without beingcombined with other elements or features. Further, an embodiment of thepresent invention may be constructed by combining parts of the elementsand/or features. Operation orders described in embodiments of thepresent invention may be rearranged. Some constructions of any oneembodiment may be included in another embodiment and may be replacedwith corresponding constructions of another embodiment. It is obvious tothose skilled in the art that claims that are not explicitly cited ineach other in the appended claims may be presented in combination as anembodiment of the present invention or included as a new claim by asubsequent amendment after the application is filed.

A specific operation described as performed by a BS may be performed byan upper node of the BS. Namely, it is apparent that, in a networkcomprised of a plurality of network nodes including a BS, variousoperations performed for communication with a UE may be performed by theBS, or network nodes other than the BS. The term ‘BS’ may be replacedwith the term ‘fixed station’, ‘Node B’, ‘evolved Node B (eNode B oreNB)’, ‘Access Point (AP)’, etc.

The embodiments of the present invention may be achieved by variousmeans, for example, hardware, firmware, software, or a combinationthereof. In a hardware configuration, the methods according to exemplaryembodiments of the present invention may be achieved by one or moreApplication Specific Integrated Circuits (ASICs), Digital SignalProcessors (DSPs), Digital Signal Processing Devices (DSPDs),Programmable Logic Devices (PLDs), Field Programmable Gate Arrays(FPGAs), processors, controllers, microcontrollers, microprocessors,etc.

In a firmware or software configuration, an embodiment of the presentinvention may be implemented in the form of a module, a procedure, afunction, etc. Software code may be stored in a memory unit and executedby a processor. The memory unit is located at the interior or exteriorof the processor and may transmit and receive data to and from theprocessor via various known means.

Those skilled in the art will appreciate that the present invention maybe carried out in other specific ways than those set forth hereinwithout departing from the spirit and essential characteristics of thepresent invention. The above embodiments are therefore to be construedin all aspects as illustrative and not restrictive. The scope of theinvention should be determined by the appended claims and their legalequivalents, not by the above description, and all changes coming withinthe meaning and equivalency range of the appended claims are intended tobe embraced therein.

INDUSTRIAL APPLICABILITY

An example in which the method and device for reporting aperiodic CSIbased on aperiodic CSI-RS in the wireless communication system asdescribed above are applied to a 3GPP LTE system has been mainlyillustrated. The above method and device are applicable to variouswireless communication systems other than the 3GPP LTE system.

1. A method for reporting, by a user equipment (UE), aperiodic channelstatus information (CSI) to a base station in a wireless communicationsystem, the method comprising: receiving, from the base station, a CSIreport-triggering message via downlink control information (DCI),wherein the CSI report-triggering message include information about atleast one CSI process among a plurality of CSI processes, and about anaperiodic reference signal resource among a plurality of aperiodicreference signal resources included in the at least one CSI process; andreporting the aperiodic CSI to the base station by updating theaperiodic CSI for the at least one CSI process based on the aperiodicreference signal resource.
 2. The method of claim 1, wherein the methodfurther comprises reporting, to the base station, user equipmentcapability information including a threshold value of CSI computingcapability of the UE, wherein when a number of CSI reports to be updatedfor a predetermined time unit exceeds the threshold value, only a numberof CSI reports corresponding to a number smaller than or equal to thethreshold value are updated by the UE.
 3. The method of claim 2, whereinwhen the number of CSI reports to be updated for the predetermined timeunit exceeds the threshold value, a report value for a number of CSIreports corresponding to a number by which the exceeding number exceedsthe threshold value is configured to be any value.
 4. The method ofclaim 2, wherein among a number of CSI reports corresponding to a numbersmaller than or equal to the threshold value, a CSI report correspondingto a lowest CSI process index is first selected.
 5. The method of claim1, wherein the method further comprises receiving information about theplurality of CSI processes including the plurality of aperiodicreference signal resources via an upper layer from the base station. 6.The method of claim 1, wherein the DCI include an uplink grant.
 7. Auser equipment (UE) in a wireless communication system, the UEcomprising: a wireless communication module; and a processor coupled tothe module, wherein the processor is configured to: receive, from a basestation, a CSI report-triggering message via downlink controlinformation (DCI), wherein the CSI report-triggering message includeinformation about at least one CSI process among a plurality of CSIprocesses, and about an aperiodic reference signal resourceamong aplurality of aperiodic reference signal resources included in the atleast one CSI process; and report the updated aperiodic CSI to the basestation by updating the aperiodic CSI for the at least one CSI processbased on the aperiodic reference signal resource.
 8. The UE of claim 7,wherein the processor is further configured for reporting, to the basestation, user equipment capability information including a thresholdvalue of CSI computing capability of the UE, wherein when a number ofCSI reports to be updated for a predetermined time unit exceeds thethreshold value, the processor is further configured for updating only anumber of CSI reports corresponding to a number smaller than or equal tothe threshold value.
 9. The UE of claim 8, wherein when the number ofCSI reports to be updated for the predetermined time unit exceeds thethreshold value, the processor is configured for setting, to any value,a report value for a number of CSI reports corresponding to a number bywhich the exceeding number exceeds the threshold value.
 10. The UE ofclaim 8, wherein the processor is configured for first selecting, amonga number of CSI reports corresponding to a number smaller than or equalto the threshold value, a CSI report corresponding to a lowest CSIprocess index.
 11. The UE of claim 7, wherein the processor isconfigured for receiving information about the plurality of CSIprocesses including the plurality of aperiodic reference signalresources via an upper layer from the base station.
 12. The UE of claim7, wherein the DCI include an uplink grant.